Aliased edge around light source in cycles
This is not a rendering artifact, everything is behaving exactly as designed.
First an explanation of why these edges appear aliased, but if all you want is to solve this issue and don't care about the background info, scroll down to the Solution.

Why it occurs
Cycles works by randomly shooting rays from the camera into the scene. It does this many times per pixel, and gets the resulting color of the pixel from the average color of samples in that pixel.
Take for example three adjacent pixels, rendered with 20 samples:

The green shape is some surface, the white part is a light source, and the red dots are the sampled points.
If the white light source has an intensity of 1.0:

The left pixel will be completely white (RGB: 1.0, 1.0, 1.0) because all the samples are also white
The middle pixel had 4 green samples and 16 white samples, meaning the resulting color will be more white than green.
The right pixel had 10 green samples and 10 white samples and so should be exactly halfway between white and green.


So because the final pixel color is the average color of the samples, what happens when we have a light source with an intensity greater than 1.0? Say, 4.0:

The left pixel will still appear white on our screens, but the actual RGB value stored there is 4.0, 4.0, 4.0.
The middle pixel will be much lighter now, because the white samples have 4x more effect on the average.
The right pixel, will also be much lighter than before for the same reason.


So if our light source is very bright (as it often has to be due to the natural quadratic falloff), the pixels on and immediately around the light will be very bright as well, since the average sample color is bright.
That's the theory, and here's proof:
A (zoomed) render of some rotated planes with varying intensity of Emission shaders:

The intensity starts from 0.5 and doubles until the second last plane (intensity of 16), then jumps to 1000.0 to show an exaggerated effect.
Below the horizontal line the intensity has been divided by 4 - so starting from 0.125, reaching 4.0 and jumping to 250.0.
Notice that the more the intensity increases, the more aliased it appears.

Solution
There's an easy way to solve this issue: Simply get the light source to appear as regular white (RGB: 1.0, 1.0, 1.0) to the camera, but still let it be brighter to non-camera rays so that it can light the scene properly.
To do this, use the Is Camera Ray output of the Light Path node as the Fac for a MixRGB node where two values are connected to the colors (the top input being the actual light intensity, and the bottom input being the intensity the camera will see):

Which results in:

But as you can see, because our light was colored it now looks a bit dull.
To fix this, we just need to use a similar light path trick for the color input to make the light appear white to the camera, but the original color to the scene:
click for full size

Note: The Light Color node in the screenshots is simply a node group containing some nodes which only allow light to be emitted from the front side of the mesh.
So there ya go! Nice anti-aliased light sources :)
The only down side of this method is if you are relying on those brighter-than-white values for compositing glare/glow - but usually the glare/glow itself will hide those nasty aliased edges.